1. Field of the Invention
This invention relates to a method of providing a silicon semiconductor pellet with a heat sink and a semiconductor device with a heat sink.
2. Description of the Prior Art
In the fabrication of semiconductor devices, bonding of silicon semiconductor pellets to heat sinks of copper tends to pose such a problem that temperature changes may fracture the silicon pellets to damage integrated circuits in the pellets. This results from the differences between the thermal expansion coefficients of the two materials. For this reason direct bonding of silicon semiconductor pellets about 1.2 mm sq. large or larger in size to heat sinks has hitherto been unfeasible.
To preclude the possibility of fracture of the silicon, it is necessary to interpose, between a silicon semiconductor pellet and a copper heat sink, a buffer such as a sheet of molybdenum whose thermal expansion coefficient is midway between those of silicon and copper, as proposed in U.S. Pat. No. 2,971,251 issued on Feb. 14, 1961 to Theo Willem Willemse, assignor to North American Phillips Company, Inc., New York, N.Y., entitled "SEMICONDUCTIVE DEVICE."
Thus, a common practice has involved pressure welding or brazing (or soldering) a molybdenum sheet to the surface of a copper heat sink, placing gold foil on the molybdenum sheet and heating them together so that gold-silicon eutectic alloy is formed thereby to connect the silicon pellet to the molybdenum. Or, the silicon pellet may be brazed or soldered to the combination of the copper heat sink and the molybdenum sheet without forming the gold-silicon eutectic alloy.
Extensive investigations have been made concerning possible causes for the above-mentioned problem. As a result, it has now been found that, with the conventional heat sinks which are made of oxygen-free copper, the heat treatment for pellet bonding completely anneals the copper and lowers its yield point with a result that the pellet bonded to the heat sink undergoes such strain at various temperatures as represented by the dashed-line curve c in FIG. 2. A subsequent heat treatment such as for nail-head bonding (at about 310.degree. C.) or some other treatments, for example for connection of the semiconductor device to a printed-circuit board (at about 250.degree. C. when soldered), produces a tensile strain above the fracture limit, causing the device to be broken or fractured.